Phonon scattering and internal friction in dielectric and metallic films at low temperatures

Abstract

We have measured the heat conduction between 0.05 K and 1.0 K of high purity silicon wafers carrying on their polished faces thin dielectric films of e-beam amorphous Si, molecular beam epitaxial (MBE) Si, e-beam polycrystalline CaF2, and MBE CaF2, and polycrystalline thin metallic films of e-beam Al, sputtered alloy Al 5056, e-beam Ti, and e-beam Cu. Using a Monte Carlo simulation to analyze the conduction measurements, we have determined the phonon mean free path within the films, and found all of them to be much shorter even than in typical bulk amorphous solids, with no exceptions. We have also measured the internal friction of these films below 10 K and found, however, their internal friction at low temperatures strikingly close to that of amorphous solids, both in magnitude and in their temperature independence, with the exception of the MBE Si and alloy Al 5056, whose internal friction is even much smaller than that of amorphous solids. The internal friction results indicate the phonon scattering in these thin films is the same as, or even much less stronger than, in other amorphous solids, according to the Tunneling Model. Thus, we conclude that the heat conduction measurements do not support the picture that the lattice vibrations of these films are glasslike, as had been surmised earlier for thin metallic films, on the basis of low temperature internal friction measurements alone [Phys. Rev. B 59, 11767 (1999)]. At the least, the films must contain additional scattering centers which lead to the very small phonon mean free path. Most remarkably, the MBE Si shows the same strong scattering of thermal phonons as do the other films, while having the negligible internal friction expected for a perfect film. The disorder causing the strong scattering of the thermal phonons in this film is completely unknown. The non-glasslike phonon scattering phenomena observed here in thin dielectric and metallic films deserve further investigations.

Elastic properties of polycrystalline Al and Ag films down to 6 mK

“…51 The lack of glassy anomalies in the specific heat and heat release might be explained in terms of the TM by an abundance of symmetric tunneling states relative to asymmetric tunneling states, 51 as would be expected if TLS form at the minima of longwavelength modulations of the kink-Peierls potential. However, the phonon mean-free path in Al films derived from heat-conductivity measurements 53 is much shorter than that predicted by the TM based on the measurements of Q −1 in Ref. 8.…”

“…Measurements of the thermal properties of plastically deformed bulk Al reveal more substantial departures from the behavior of most amorphous solids. 53 The thermal phonon mean-free paths in pure e-beam deposited Al films and in 2.5 mm diameter, 5% deformed 5N bulk Al were determined from heat conduction measurements between 0.05 and 1.0 K. These measured mean-free paths are much smaller than those predicted by the TM based on the measured Q −1 in the temperature-independent region. In the case of the films, the discrepancy was greater than an order of magnitude.…”

“…Pohl and co-workers 8,51,53 have studied the effect of plastic deformation on the acoustic and thermal properties of bulk Al. The plastic deformation is thought to produce a dislocation density n disl = ͑10 11 cm −2 ͒⑀ plas , where ⑀ plas is the plastic strain.…”